Typical emulsion coating operations require liquid kettle blending to smooth batch-to-batch variability during long coating events. Conventional continuous liquefaction technology requires quick chilling, and/or pelletization, and solid blending equipment to pre-blend the emulsion in solid form. This technology is described in U.S. Pat. No. 5,182,190.
U.S. Pat. No. 5,523,537 describes a modified passive liquefaction system. In this system, a liquefying apparatus having a hopper which includes an upper and lower section is described. The upper section is adapted to receive a meltable solid material and has a coiled tube disposed therein. By heating the coiled tube, solid material supported thereon is melted and allowed to pass therethrough such that the liquefied product is stored in the lower section of the hopper and subsequently drawn off. Use of this technology with U.S. Pat. No. 5,182,190 allows one to first blend the solid material, and then passively liquefy the gelled material.
In a typical sensitizing operation for producing photographic films, "long" coatings require the use of many batches of product to produce a continuous coating. "Long" refers to the length of time required to perform the operation. A long coating in a film/paper plant can be a two to three day operation. In dedicated plants a long coating operation may last seven to ten days. It is not uncommon that the sensitometric parameters between batches are variable. If each batch is coated sequentially and in an end-to-end manner, unacceptable variations to the coated product can be produced. The variations in the sensitometric parameters of consecutive batches can result in step changes in the coated product which cannot be compensated for fast enough by a metered dye feedback system. Attempts to overcome this problem have included methods such as blending and melting, blend kettles, and modified passive liquid in-line segmented blending as is taught in U.S. Pat. No. 5,374,120. With melt/blend kettles, two or more product batches are added to the melt kettle and liquefied. The resulting homogeneous solution can then be delivered to the coating operation. By adding the same components to each subsequent melt kettle, a continuous blend is produced. Blending can also be achieved by incorporating a blend kettle with two melt kettles. The blend kettle is utilized as a reservoir with solution being supplied sequentially from the two melt kettles. These methods have the drawbacks of long melt hold time (melt drift) and significant amounts of liquefied solution (waste). Melt drift refers to a drift in the sensitometry of a melt over time. Photographic emulsions tend not to be particularly stable and as a result of liquid hold times of up to seven hours in a melt kettle, emulsion chemistry changes due to the solution temperature which, in turn, yields a change in the sensitometry.
The modified passive liquid in line segmented blender described in U.S. Pat. No. 5,374,120 is designed to blend one to four batches of product by simultaneously liquefying the batches into a surge pot. The solutions are blended within the surge pot by means of a kettle type mixer. The process achieves blending by liquefying the emulsion in each of the four quadrants. The liquefaction rate of each quadrant is roughly equal. Total liquefaction of all four quadrants is dependent on coating machine demand. Transitions are achieved in four steps of 25% each and independent solution flow rates cannot be regulated. The segmented blender requires hand loading a product into each quadrant and is labor intensive.
There remains a need for an improved melting/blending operation that eliminates sensitometric issues such as melt drift associated with melt hold, greatly reduces the amount of waste of liquefied product at any one time, and allows for bulk material handling. Further, it would be advantages to have a melting/blending system that allows for greater flexibility in the blending process.